1. Field of the Invention
The present invention relates to a magneto-impedance element and a method of manufacturing the same. The magneto-impedance element serves as a magnetic head which is used for magnetic storage and magnetic recording in a computer, an information machine, etc., and is also used in control of drive apparatuses therefor. The magneto-impedance element also serves as a magnetic sensor head for detecting magnetic fields, currents, and the like.
The present invention also relates to a method and apparatus for magnetic sensing using a magneto-impedance element.
The present invention is applicable to many fields such as computers, information machines, information home appliances, mechatoronics, power electronics, medical electronics, industrial instrumentation such as non-destructive inspection, environmental measurement, and scientific measurement utilizing terrestrial field or rock magnetism.
Specifically, the present invention is applicable to all industrial fields in which Hall elements, magneto-resistance (MR) elements, and flux gate sensors are currently utilized, because the magneto-impedance element can be miniaturized to the same degree as Hall elements and MR elements, and the sensitivity in detecting magnetic fields is 100 fold or more compared to Hall elements and MR elements, i.e., comparable to that of flux gate sensors for detection of a uniform field. Therefore, in these areas, the present invention contributes to giving intelligence to measurement and control.
2. Description of the Related Art
The present inventors have proposed such a magneto-impedance element, as disclosed in Japanese Patent Application Laid-Open (kokai) No. 7-181239 entitled "Magneto-Impedance Effect Element".
The element disclosed in the publication is a slender magnetic element which is made of zero-magnetostriction amorphous wire or the like and whose circumferential permeability .mu. is high. When a high frequency current is caused to flow through the element, an AC voltage appears between both ends thereof, and the amplitude of the AC voltage sensitively varies in response to a magnetic field applied externally in the longitudinal direction of the element. In detail, when the circumferential permeability .mu. changes due to an external magnetic field, the impedance Z of the element changes due to a skin effect while maintaining the relationship .vertline.Z.vertline..infin..sqroot..mu. (the impedance decreases at a relatively low frequency, and increases at a high frequency). Therefore, the above-described element is called a "magneto-impedance element" or an MI element.
In the case where an amorphous wire of FeCoSiB which has been drawn and tension annealed so as to have a diameter of 30 .mu.m and a length of 1 mm is prepared as an MI element and a current of 5 mA having a frequency of 1 MHz is supplied to the MI element, the impedance .vertline.Z.vertline. of the MI element decreases by about 50% when an external magnetic field H.sub.ex of 5 Oe is applied (i.e., 10% decrease per Oe). Therefore, the MI element has a sensitivity about 100 times that of a conventional MR element (the resistance variation ratio of an MR element is about 0.1% per Oe), although the size is as small as the MR element. This means that the MI element is a micro magnetic element having a super high sensitivity. However, the impedance .vertline.Z.vertline. symmetrically varies depending on the absolute value of an external magnetic field H.sub.ex and does not depend on the sign of the external magnetic field H.sub.ex. For this reason, as in the case of an MR element, the MI element cannot form a linear magnetic sensor as is.
To overcome this problem, two MI elements are used and opposite bias magnetic fields are applied to these elements. By detecting the difference between the output voltages of the two elements, a magnetic field can be detected with high sensitivity. Therefore, a highly sensitive magnetic sensor can be obtained which has a resolution of 10.sub.-5 Oe and which has a sensitivity comparable to a flux gate sensor for a uniform magnetic field. Since the head length of the MI element can be shorted to about 1 mm, the MI element can detect a local magnetic field with considerably high sensitivity. When a resonant type multivibrator is formed by combining an MI element and a bipolar transistor or an FET, oscillation of 100 MHz-200 MHz can be easily obtained. Therefore, in the case of an amplitude modulation type magnetic sensor, a super high response speed is obtained because of a very high cut-off frequency of about 10 MHz. The cut-off frequency of a flux gate sensor is about 1 kHz.
As described above, the MI element is a basic element for constituting a micro magnetic sensor having a super high sensitivity and a super high response speed. However, the MI element has drawbacks which must be resolved. One of such drawbacks is a manner of applying bias magnetic fields. Conventionally, the following three methods have been used for applying a DC bias magnetic field:
(i) a coil is wound around a head, and a direct current is supplied to the coil; PA1 (ii) a permanent magnet is disposed in the vicinity of a head; and PA1 (iii) a coil is wound around a soft magnetic material having a rectangular BH characteristic, and a direct current is supplied to the coil, whereby a DC magnetic field generated from magnetic poles of the soft magnetic material is applied to the head.
However, these methods prevent the MI element from being manufactured in a micro size, and make the adjustment troublesome. Additionally adjustment of a long wire head having a length of a few tens of centimeters to detect a large current, is quite troublesome.